is there anny real differnce in engine kits?

Dax and Arrow (and CR machine now) are the only engines I'm aware of that actually do anything more to the engines other than up the price. None of the "upgraded" kits I've seen were worth a penny more than the ebay kits.
Dax has a great reputation here with his balanced bottom end motor, Arrow builds sweet (expensive) stuff too. Don't know much about fred (cr machine) but his stuff looks pretty nice...and at the prices he charges, they should be!
Unless you buy from these three, there is no or very little difference in quality from kit to kit as far as I've seen.
I've built at least 30 or 35 kits from many different vendors. I buy strictly from rose 326 and mingsinternational now. (same vendor)
IF I ever decide to build a better quality engine, it will start with a Dax bottom end.
Just my opinion. You will hear many who disagree.

Having used an "out of the box" grubee kit, boygofast kit, and now a fred race motor.... there are MAJOR quality gaps.

grubee kits on a whole are decent quality, some kits have a ****ty carb design - but they are kinda hit or miss with getting a straight crank. friend and i bought 3 kits, 2 from one batch, one from another. the 2 from the same batch were (and mine still is) great. 43.62mph with mods on mine. the 3rd vibrated like a banshee past 4500rpm (ish)

boygofast kits are decent, they seem to be a touch faster top end out of the box than grubee kits, may be from the 40mm stroke vs 38mm on the gt5 grubee. still kinda iffy with straight cranks

and finally, the FREDMOTOR. My buddy has his installed and it did 40 out of the box on a 36t. still hafta dial in the carb, but thats not a bad start. its SUPREMELY smooth running. Sounds great. LOOKS great, head is beautifully machine, EVERY single bolt is replaced with uprated hardware, the clutch is properly lubed and adjusted, everything is perfectly sealed. We did a 92.6 mile trip the day we installed his motor - not a single problem. could be more fuel efficient, but i think he needs to downjet it.

long story short.... if moneys no object, look at a fred motor. i gave my friend **** about buying one until it arrived. needless to say i've got my own fred motor in a box .

in the meantime Im having cranks balanced and straightened so i can build my own critters!

there are diffs even between two kits ordered same day from same place - most are 'acceptable' tho